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Peripheral Nervous System
Nervous system consists of: CNS = brain and spinal cord ~90% (90 Bil) of all neurons in body are in CNS PNS = Cranial nerves and spinal nerves, nerve plexuses & ganglia ~10% (10 Bil) of all neurons in body are in PNS PNS is our link to the outside world without it CNS us useless sensory deprivation hallucinations some terminology: CNS PNS bundles of axons tract nerve cell bodies, dendrites, synapses nuclei ganglia Nerves each nerve is an organ composed mainly of nervous tissue (neurons and neuroglia) and fibrous connective tissue with rich supply of blood vessels arranged in pattern similar to that of muscle organs: endoneurium around each individual neuron perineurium around bundles of neurons (=fascicles) epineurium around entire nerve 2 kinds of neurons can be found in nerves: sensory (afferent) neurons ~2-3M; 6-8x’s more sensory than motor fibers motor (efferent) neurons ~350,000 efferent fibers somatic motor neurons autonomic motor neurons Nerves can be classified according to the kinds of neurons they contain:
a. sensory nerves – contain mainly sensory neurons b. motor nerves – contain mainly different kinds of motor neurons c. mixed nerves – contain a combination of both ganglia = groups of cell bodies and sometimes dendrites and synapses associated with nerves of PNS examples of PNS ganglia: dorsal root ganglia = cell bodies of sensory neurons autonomic chain ganglia = cell bodies, dendrites & synapses of autonomic motor neurons nerve plexuses weblike interconnections of fibers from many nerves eg. spinal nerve plexuses several spinal nerves come together eg. autonomic plexuses PNS consists of 43 pairs of nerves branching from the CNS: 12 pairs of cranial nerves 31 pairs of spinal nerves Cranial Nerves 12 pairs of cranial nerves structurally , the cranial nerves originate from: cerebrum I, II midbrain III, IV pons V, VI, VII,VIII (pons/medulla border) medulla IX, X, XI, XII functional classification of cranial nerves : a. sensory cranial nerves I. Olfactory [sense of smell] II. Optic [sense of sight] VIII. Vestibulocochlear [senses of hearing and balance] has a few motor fibers
Dermatomes sensory neurons of each spinal nerve innervate the skin and skeletal muscles in the roughly same order in which they emerge from the spinal cord detailed mapping of the skin surface reveals a close relationship between the source of nerve fibers and the location (superior to inferior) of the skin segments each innervates segmental arrangement of spinal nerves this is clinically useful since physicians can determine the site of spinal damage by simple pinprick exam Spinal Nerve Plexuses after the spinal nerves exit the intervertebral foramina they branch and interconnect to form plexuses from these plexuses new nerves emerge that contain a mixture of fibers from various spinal nerves Cervical Plexus formed from C1 – C4, supplies sensory and motor neurons to head, neck and upper shoulders emerging nerves include: phrenic nerve (C3-C5) diaphragm Brachial Plexus formed from fibers in C5 to C8, & T innervates shoulders and upper limbs emerging nerves include: axillary (C5,C6) to deltoid radial (C5-C8,T1) triceps and forearm extensors median (C5-C8,T1) flexor muscles of forearm and hand ulnar (C8,T1) wrist and hand muscles this plexus is sometimes stretched or torn at birth leading to paralysis and numbness of baby’s arm if untreated may produce “withered arm”
prolonged use of crutch may injure this plexus= crutch palsy [most thoracic spinal nerves (2-12) do not form a plexus] Lumbar Plexus formed from fibers in L1 to L innervates abdominal wall, genitals, parts of leg emerging nerves include: femoral nerve (L2-L4) thigh and leg muscles Sacral Plexus formed from fibers in L4 & 5, S1 to S supplies nerves to buttocks, perineum, leg emerging nerves include: sciatic nerve (L4,L5, S1-S3) leg muscles; largest nerve in body
Autonomic Nervous System
2 major subdivisions of the motor neurons of the PNS somatic - innervate skeletal (voluntary) muscles autonomic – innervate smooth and cardiac (involuntary) muscles and glands autonomic = “self governed” autonomic nervous system consists of motor fibers that innervate the visceral organs; organs that function automatically ANS tends to regulate visceral effectors in ways that tend to maintain or restore homeostasis
III (oculomotor) VII (facial) IX (glossopharyngeal) X (vagus) and fibers in some sacral (S2-S4) spinal nerves no chain ganglia, fibers not interconnected ganglia are usually near organs they innervate Functions of ANS Branches Sympathetic adapts body for intense physical activities: increases alertness, blood pressure, air flow, blood sugar concentrations, blood flow to heart and skeletal muscles acts as an emergency system emergency or stress that threatens homeostasis “fight or flight” maximum energy expenditure acts as a unit = mass activation more diffuse, body-wide response effects are longer lasting Parasympathetic most active in non-stressful, non-emergency situations “resting and digesting” tends to have a calming effect on body: reduced energy expenditure and normal body maintenance organs are individually activated no mass activation short lived, localized effects promotes normal daily activities: GI tract works to process food
glandular secretions
peristalsis blood pressure, heart rate, respiratory rates maintained at low levels Interactions between two branches of ANS the body doesn’t alternate between only sympathetic or parasympathetic activity normally, both systems are active both always exhibit at least a baseline level of “ autonomic tone ” eg. parasympathetic always maintains smooth muscle tone in intestine and keeps heart rate down to 70 bpm (vs intrinsic 100 bpm) eg. sympathetic always maintains smooth muscle tone around most blood vessels to maintain blood pressure most visceral organs receive dual innervation of both branches of ANS in organs with dual innervation can be antagonistic cooperative some organs lack dual innervation and there is no interaction Autonomic Control Centers many autonomic reflexes have been discussed earlier when discussing Brain but regulation of ANS is far from being completely automatic as implied earlier there is a hierarchy of control of autonomic effectors Autonomic Centers in Cerebral Cortex (frontal lobe) Autonomic Centers in Limbic System Autonomic Centers in Hypothalamus
